The genomic structure of murine type-C RNA leukemia viruses has been studied using the Friend strain of the replication-defective spleen focus-forming virus (SFFV) and the helper independent Friend leukemia virus (F-MuLV) which can cause a rapid erythroleukemia in appropriate mice. Two proteins encoded by SFFV have been identified, a gag gene related protein and an env related protein; however, there is no direct evidence to demonstrate which protein(s) of SFFV or of FMuLV is responsible for the leukemogenicity. This evidence can be obtained by genetic analysis of the viruses. A comparison between the viral RNA of FMuLV and that of a non-leukemia inducing variant of FMuLV has been made which indicates that 15-20% of the genome is altered. This altered area can now be mapped on the genome to determine which genes are involved in the leukemic potential of FMuLV. Also, the genome of FMuLV has been molecularly cloned and results obtained in the last year will allow the cloning of the SFFV genome and of specific DNA fragments which will be invaluable tools in studying the genetics of these erythroleukemia inducing viruses.